Field of the Invention
Embodiments of the present invention generally relate to an apparatus and a method of growing graphene on various substrates for different applications.
Description of the Related Art
Graphene is a flat monolayer of polycyclic carbon atoms arranged into a quasi two-dimensional (2D) honeycomb lattice of mostly sp2 bonds, and is a basic building block for graphitic carbon material of other dimensionalities. Graphene can be wrapped into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphites. Graphite is generally not useful for electronic devices, whereas graphene, with its intrinsic semi-metal and zero-gap electric properties, tunable band gap, and strong mechanical strength (as one of the strongest materials ever tested), is a suitable material for integrated circuit (IC) fabrication (e.g., for constructing quantum computers using anionic circuits). Graphene is proposed to be used in many different applications, such as gas sensors, nano-electronics, interconnects, transistors, transparent conducting electrodes, ultracapacitors, solar cells, ITO replacement, engineered piezoelectricity, diffusers, distillation of ethanol, membrane-type devices, graphene nano-ribbons, graphene optical modulators, graphene bio-devices, and many others. However, graphene is currently only grown in the lab and no efficient process for making graphene exists, making it very expensive to grow. For example, graphene produced by exfoliation is among the most expensive materials on Earth.
Graphene can be grown in a CVD or epitaxial process onto a metal-containing catalyst surface (e.g., substrates with nickel or copper on their surfaces to seed the growth of graphene) using a gaseous carbon source that requires very high deposition temperatures (e.g., 900° C. or higher) and is difficult to grow directly on silicon substrates, and graphene grown on metal and transferred to non-metal substrates requires meticulous surface attachment in many cases to achieve conductivity. Graphene has been shown to grow on silicon carbide substrates using an epitaxial process or a silicon evaporation process, but the temperature has to be higher than 1,000° C. Graphene may also be formed by reduction of graphene oxide sheets at high temperatures. These applications of growing graphene films are not suitable for most device fabrication due to thermal budget requirements. For example, substrates for CMOS devices typically have a temperature threshold at about 400° C.
Furthermore, even at high temperatures, current CVD or epitaxial graphene growing processes require long deposition times, due to low reactivity of the gaseous carbon sources used and low efficiency in incorporating reactive species into a growing graphene film. Further, CVD or epitaxial-deposited graphene films are not uniform, resulting in randomly oriented grains at sizes less than one square millimeter (mm) and varying numbers of graphene layers. Growth is often nucleated at a number of locations simultaneously, contributing to the formation of randomly oriented grains in a graphene monolayer/sheet.
Assembly of the gaseous reactive species on the surface of a substrate in a manner that is conducive to a monolayer of graphene growth is a major challenge. The gaseous precursors participate in a number of side reactions resulting in a loss of reactive species, polymerization with unwanted functional groups, and undesirable side products, all of which decrease the number of active precursor molecules available for graphene growth. Thus, there is a need to find a wider range of precursors and source compounds for growing graphene.
Graphene's electric properties are strongly linked to its thickness and length. For example, a graphene monolayer is less than 1 nm thick, typically at about 3 angstroms, as compared to the thickness of a semi-conductor film generally between 150 angstroms to 5000 angstroms. Most graphitic carbon films are grown to a thickness of 100 nm or thicker. In some such films, patches or spots of graphene at a length of about 1-2 microns have been observed. Such patches are not long enough for device applications, however, and it is difficult to grow graphene that covers the whole surface of a substrate (e.g., a silicon wafer).
Therefore, there is a need to develop a low temperature process, to find new precursors, and to grow high quality graphene films on a larger scale for graphene's many industrial applications.